Hot melt adhesives are solid at room temperature but, upon application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. Curable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. Advantages of hot melt curable adhesives over traditional liquid curing adhesives are (1) their ability to provide “green strength” upon cooling prior to cure and (2) they provide adhesives of very low crosslinking density and thus high levels of flexibility and toughness.
The majority of reactive hot melts are moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane/urea polymer. Polyurethane prepolymers are conventionally obtained by reacting diols with diisocyanates. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the adhesive and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid which is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a lightly crosslinked material held together primarily through hydrogen bonding, urea groups and urethane groups.
Silane-based moisture curing adhesives have several advantages over isocyanate-based adhesives. These advantages include: 1) no bubbling in the bond line since the by-product of the curing reaction is not a gas (instead of CO2, ordinarily methanol or ethanol is released), 2) increased flexibility after crosslinking since siloxane bonds are more flexible than urea bonds which strongly hydrogen bond to each other, 3) no hazardous isocyanate vapors, and 4) improved adhesion to glass and other surfaces through reaction with surface hydroxyls. Hot melt adhesives incorporating silane crosslinking groups have been developed based on a number of solid polymers. Grafting of silane reactive groups onto polyolefins (H. G. Wey, Munich Adhesives and Processing Seminar 1997), unsaturated styrenic block copolymers (WO 91/06580), polyurethanes (U.S. Pat. No. 6,749,943), and ethylene vinyl acetate copolymers (GB 2,197,326) have had limited commercial success, likely due to the inability of the silane groups to condense with sufficient rapidity when attached to a solid polymer. In addition, transesterification of silanol groups with ester groups has limited the choice of catalysts (U.S. Pat. No. 6,749,943).
A need exists for improved hot melt adhesives based on silane cure. The present invention addresses this need.